Abstract

Male masturbation or sexual-auto stimulation is well documented in humans. This
study dealt with the occurrence of masturbation in non-human primates in general, and in
Japanese macaques (Macaca fuscata) in particular, from the perspective of behavioural
ecology.
In an interspecific comparison of 52 primate species, male masturbation was found to
be a common behaviour that correlates more strongly to species that exhibit a multi-male
multi-female breeding system than to species living in monogamous, one-male units, solitary
or dispersed breeding groups. This result is in direct contrast to established theories of sperm
competition, which predict that species with a higher risk of sperm competition (i.e. those
with multi-male multi-female breeding systems) should invest in large ejaculate volumes so
as to out-compete conspecifics. They should not, therefore, produce and seemingly waste
ejaculate by masturbating.
In wild living (on Yakushima Island) and captive Japanese macaques, a multi-male
multi-female seasonal breeding species with a high incidence of female and male
promiscuity, two hypotheses concerning how masturbation may be beneficial to males in
regard to sperm competition were tested. First, I tested whether or not males flush out low
quality sperm from their genital tracts when they masturbate. Second, I tested for whether or
not the subsequent ejaculate was "fresher".
The mechanism of how ejaculate parameters change in response to storage time in the
male genital tract was determined by performing experiments with singly-caged Japanese
macaque males. I found that the longer an ejaculate was stored, the larger its volume and total
sperm number became. However, the longer an ejaculate was stored, the lower the sperm
swimming velocity and percentage of vital sperm became.
Based on this result, roughly two types of ejaculates could be distinguished and
correlated with commonly known male mating strategies. Guarders tend to be older, high
ranking males who have the opportunity to perform many consecutive matings with the same
female. They bring into sperm competition games Type A ejaculates, which have been stored
for more or less lengthy periods and are characterised by a large volume and a large total
sperm number comprised of slow swimming and many dead sperm. In contrast, sneakers,
tend to be younger males of middle or low rank, who are usually able to mate only
opportunistically (almost never consecutively and almost always covertly) and their
ejaculates inevitably face sperm competition from guarders’ (or from other sneakers’) ejaculates. Sneakers masturbate before mating and thus bring into the female only a small
volume of ejaculate but one with fast moving sperm, all of which are a live (Type B). In
contrast, guarders exclusively masturbate out long stored ejaculate only on days when there
are no estrus females available in the troop. Guarders never were seen to masturbate before
mating.
Masturbation, then, is one mechanism by which lower ranking males attempt to make
the best of their limited mating opportunities in a species characterized by high levels of
sperm competition by investing in ejaculate quality as opposed to quantity. This result is
supported by a DNA-paternity-exclusion analysis in the study troop on Yakushima Island, in
which six of nine babies were sired by sneakers. Masturbation is, thus, physiologically
adaptive to sperm competition in primates. Both, guarders and sneakers use masturbation to
improve ejaculate quality. Therefore, masturbation can be regarded as an evolutionarily
stabilised strategy (ESS).